1. Field of the Invention
The present invention relates in general to an image forming apparatus adopting an electrostatic system, an electrophotographic recording system, or the like. In particular, the present invention relates to an image forming apparatus for forming an image such as a dot pattern other than a normal image on an image bearing body in order to prevent an image defect when a developer image on an image bearing body is transferred onto a transferring medium such as an intermediate transferring body or a transferring material.
2. Related Background Art
Heretofore, there has been used an image forming apparatus having a plurality of image forming units as recording units in each of which a laser beam or a light emitted from a light emitting element such as an LED which is light-modulated in accordance with external information is applied to an image bearing body such as a photosensitive drum having a surface charged with electricity corresponding to a predetermined potential to change the potential of a portion irradiated with the light to thereby form an electrostatic latent image, the electrostatic latent image on the photosensitive drum is developed, and a developer image (toner image) is transferred onto a transferring medium such as a transferring material or an intermediate transferring body conveyed by a transferring material conveying body. Regarding such image forming apparatus, there has been proposed an image forming apparatus capable of forming a color image by utilizing a method in which: respective image forming units form images having different colors, and the images are transferred onto the transferring materials one on the other while transferring materials are successively conveyed to the recording units; or after the images are transferred onto an intermediate transferring body one on the other, the images are collectively transferred onto a transferring material.
Here, a primary transfer system in which an image is transferred from a photosensitive drum to a belt-like intermediate transferring body (intermediate transferring belt) will be described as an example. In an image forming apparatus of this sort, it is conceivable that in particular, for the purpose of enhancing a primary transfer latitude, a primary transfer current is optimally set. However, when the primary transfer current is low, transfer deficiency is caused, while retransfer is caused when the primary transfer current is high.
For this reason, for the purpose of realizing the enhancement of the primary transfer latitude, a method including providing a difference in peripheral speed between each of the photosensitive drums and the intermediate transfer belt, is suitably implemented. This provision of the difference in peripheral speed results in that in particular, a central portion of a fine line of a secondary color is not omitted to realize enhancement of the transfer latitude. However, a frictional force is usually generated between each of the photosensitive drums and the intermediate transferring belt due to the difference in peripheral speed.
A coefficient of friction is changed between a case where there is developer (toner) between each of the photosensitive drums and the intermediate transferring belt and a case where there is no developer between each of the photosensitive drums and the intermediate transferring belt due to the frictional force generated between each of the photosensitive drums and the intermediate transferring belt, so that a rotational speed of each of the photosensitive drums fluctuates. This results in that image exposure to the photosensitive drums is blurred and thus an image streak is generated.
This phenomenon also occurs in a transfer system for transferring a toner image onto a transferring material conveyed from the photosensitive drums to the transferring material conveying body. In this case, the transferring material conveying body and the intermediate transferring body are collectively referred to as a transfer/movement unit.
A problem is described in Japanese Patent Application Laid-open No. H11-52758 (or U.S. Pat. No. 6,091,922) that with a construction in which no difference in peripheral speed is provided between an image bearing body and a transfer/movement unit, a non-intentional speed difference is generated due to decentering or the like of a drive roller, and as a result, color drift is generated. Further, in Japanese Patent Application Laid-open No. H11-52758 (or U.S. Pat. No. 6,091,922), there is described a construction in order to solve this problem. That is, dot toner images are dispersedly formed in the form of predetermined minute dots so as to overlap a normal image, so that an image is more stably formed to allow an image of high quality to be printed.
However, in many cases, such a dot pattern is formed so as to overlap a toner image as a normal image, which is intentionally formed by a user based on external information. Thus, there arises a first problem in that if each dot toner image is formed in a state of laser full lighting using the above-mentioned image forming apparatus when a predetermined dot pattern is formed, even when a dot toner image is formed with yellow toner so as to be made inconspicuous, the dot toner image formed with yellow toner becomes conspicuous in a portion (white background portion) to which no toner is transferred in the toner image formed on the basis of the external information.
Further, there arises a second problem in that when conversely, a dot pattern is formed not in a state of the laser full lighting, but in a halftone state, since a color of the dot pattern appears to be missing in a set-solid portion in the toner image formed on the basis of the external information, a normal image appears to be rough to reduce the quality of the image.
Moreover, generation of an image streak due to a change in frictional force is not limited to only a case of the formation of a color image as a normal image. Thus, for example, in a case where an image of black (K) monochrome is formed as a normal image by a color copying machine, or even in a case where an image is formed by a monochrome copying machine, a light and shade image streaks appear due to a change in frictional force. For this reason, in formation of an image of black monochrome as well as in formation of an image by the monochrome copying machine, there is a need to form a dot pattern with black toner. However, when a dot pattern is formed with black toner, for both the above-mentioned first and second problems, the disadvantages such as the conspicuousness and the roughness of the dot pattern are emphasized as compared with a case where a dot pattern is formed with yellow toner. As a result, the reduction of image quality becomes a serious problem.